1. Field of the Invention
The present invention relates to an exposure apparatus and a method of manufacturing a device using the same.
2. Description of the Related Art
An exposure apparatus uses TTR (Through The Reticle) measurement as a method of relative alignment between a reticle (original) and a wafer (substrate). In the TTR measurement, the relative position between a reticle-side measurement mark formed on a reticle or a fiducial plate placed in its vicinity, and a wafer-side measurement mark formed on a wafer or a fiducial plate placed in its vicinity is measured via a projection optical system. It is a common practice in the TTR measurement to use, as measurement light, light containing the same wavelengths as exposure light.
The TTR measurement can also be used to measure various kinds of parameters by measuring a plurality of points within a region to which a pattern is projected by a projection optical system. Rotational components and a magnification, for example, can be measured by measuring the relative position between two points within a plane perpendicular to the optical axis of a projection optical system. Distortion can also be measured by increasing the number of measurement points. Similarly, the tilt and curvature of field of the image plane of a projection optical system can be measured by measuring the positions of measurement points in the optical axis direction (see Japanese Patent Laid-Open No. 2005-175400).
The resolution limit (critical dimension (CD) or minimum feature size) of an exposure apparatus is proportional to the wavelength of exposure light, and is inversely proportional to the numerical aperture of a projection optical system. Hence, an exposure apparatus has been developed by shortening the exposure wavelength, and increasing the NA of a projection optical system in order to improve the resolution limit. However, the depth of focus of a projection optical system is proportional to the wavelength of exposure light, and is inversely proportional to the square of the numerical aperture of the projection optical system. Therefore, the depth of focus is rapidly decreasing with an increase in resolution of an exposure apparatus, and a demand for higher accuracy of focusing performed based on TTR measurement is becoming stricter.
Also, since the device line width narrows with an improvement in resolution limit, a demand for higher accuracy of alignment performed within a plane perpendicular to the optical axis based on TTR measurement is becoming stricter as well.
To meet these demands, an image obtained by projecting a mark for use in TTR measurement onto the image plane of a projection optical system is desirably similar to an image of a device. Hence, the line width of a mark for use in TTR measurement is narrowing as well.
On the other hand, an exposure apparatus must have a high throughput in order to produce devices in large quantities in a short period of time. This makes it necessary to shorten the time taken for TTR measurement. In TTR measurement of a plurality of points, the measurement time can be shortened by simultaneously measuring two or more measurement points. Simultaneous measurement of a plurality of points can be done by setting a plurality of pairs of a reticle-side measurement mark and a wafer-side measurement mark (see Japanese Patent Laid-Open No. 2008-53618).
Unfortunately, simultaneous measurement of a plurality of pairs of marks is becoming difficult owing to, for example, a reduction in detection range, which accompanies miniaturization of marks for use in TTR measurement; manufacturing errors of the marks; and a shift in imaging position due to a change in imaging performance of a projection optical system, which accompanies exposure. It is becoming difficult to, for example, perform TTR measurement of one pair of a reticle-side measurement mark and a wafer-side measurement mark, while performing TTR measurement of another pair of a reticle-side measurement mark and a wafer-side measurement mark.